Magnesium



Patented Aug. 14, 1945 UNITED STATES PATENT. orrica MAGNESIUM HenryFord. Dearborn, Elbert Edwin Ensign, Ypsilanti, and Archibald 0. Quinn,Deal-born, Mich, assignorl to li'ord Motor Company, Dearborn. Mich, acorporation of Delaware Application February 16, 1942, Serial lilo.431,078

13 Claims.

This invention relates to a process of producing metallic magnesium fromthe ores of that metal by dissociation under conditions of comparativelymoderate temperature and high vacuum.

An object of this invention is the production of metallic magnesiumdirectly in reguline iorm under controlled termperature and pressure asdistinguished from the present commercial carbothermal reduction processin which the high temperatures oi the electric are are utilized atpressures of atmospheric or greater.. Another object is to make possiblethe use of the common and readily available ores including magnesiumcarbonates, oxides and the dolomites. A further object of this inventionis to form gases during the course of the reaction and in situ of theinitial formation oi magnesium to control the relative partial pressuresobtaining in the system to obtain optimum magnesium productio A featureof this invention is that it comprehends the dissociation of. themagnesium-bearing material by heat alone rather than reduction by anintermediary reducing substance. IA! a corollary, the process increasesthe rate of dissociation at the desired temperatures, supplies gases atthe points of dissociation to control the partial pressures, andcombines the dissociated oxygen in forms which are essentially inertwith the magnesium formed at the temperatures prevailing in the furnaceor in the condensing chamber.

The reduction of magnesium oxides with carbon has been commerciallypracticed in the high temperatures prevailing in an electric arc. Thediiii- With these and other objects in view, the invensteps there- 01,described in this specification; claimed in the tion consists oi theprocess and the claims, and illustrated in the accompanying drawing inwhich:

Figure 1 is a diagrammatic representation of source of the carbonateradical, and a quantity 01' carbon, all of these materials beingintimately mixed in a finely divided state. Preferably all of them, butat least the carbonate, must pass a 200- mesh screen and are then formedin briquettes.

The briquettes are placed in a closed metallic chamber which is heatedwhile vacuum is applied until conditions of pressure below 3000 micronsand of temperature in the range from 2000 to 2500 Fahrenheit areobtained. As a magnesium source, magnesium oxide (MgO), magnesiumcarbonate (MgCOs), dolomite (MgCOa-CaCOa), partially calcined dolomite(MgCOa-CaO), or calcined dolomite (MgO- CaO) may be used; but as it isessential that there other economically feasible carbonate which dis-.

culty with this procedure is that the pressures prevailing areatmospheric or greater; and,-as at these pressures the reductionreaction is readily reversible, the magnesium vapor formed must besharply quenched to the solid form to prevent reversion. The quenchedmagnesium is in the form of a pyrophoric .powder which must be protectedfrom contact with air. Certain disadvantages are encountered in thisquenching, and further processing of the magnesium powder is required toreduce it to the reguline form.

The present invention avoids the diiliculty in teaching a method ofreducing the reversibility of the dissociation and its immediate Productis v nesium oxide or magnesite, this primary decompo-l metal in acrystalline or reguline form. It permits the operation to proceed attemperatures at which metallic containers may be used and the componentsubstances placed under vacuum. It is, in essence, a one-step process,which may be performed either by the batch method or continuously withapparatus within the scope of good engineering practice.

be a source or the carbonate radical, ii MgO or MgO-CaO be used, acarbonate must be added:

Figure 1 indicates this source as R carbonate audit is .to be understoodthat this may be either magnesium carbonate, calcium carbonate or anysociates when heated and whose resulting oxide doa not flux at thetemperatures obtaining.- If finely divided magnesium carbonate is usedas the magnesium source, it decomposes, as explained later, only atrelatively high temperatures to form magnesium oxide and carbon dioxide-On the other hand, it the source is originally magsition is avoided. Inany event there must be a finely divided carbonate which decomposes toterm the appropriate oxide and the carbon dioxide. Therefore, as theheating continues, relardless of the original materials, the retort willcurtain solidmagnesium oxide, nondecomposed carbonate, carbon dioxide, Roxide and carbon. It may be shown that the magnesium oxide willdissociate without the intervention or any reducing or catalytic agentat the temperature and pressure now prevailing, but this dissociationproceeds at a very slow rate and unless the om and magnesium areseparably removed from the 5 dissociation zone, reversal takes place.Even the membered that it is very dimcult to obtain retorts which willwithstand the stresses when under vacuum at temperatures above 2100 F.Even Nichrome containers fail readily and the avoidance of even thesmallest temperature increment is economically desirable, as reflectedin longer retort life.

At this point, reference is made to Figure 2, which shows the computedtemperature-pressure equilibrium relations obtaining in the magnesiumoxide and carbon reaction. It will be understood that specific valueswill differ when other materials are used and in the presence of othergases, but that the general conditions remain the same and the curvesmay be taken as indicative of these. Curve A shows the equilibriumcondition as expressed in the partial pressure of magnesium vaporagainst temperature. It may be shown that the partial pressure of the Cmust at least equal the vapor pressure of magnesium and accordinglycurve B represents the permissive total pressure obtaining in the systemfor equilibrium condition were no other gases present. Therefore, as ageneral rule, at all points above curve B at least som magnesium will bedissociated and present as a gas.

To hasten-dissociation and make the process economically feasible, ithas been found that interface reactions must be encouraged between thesolid components, the partial pressures controlled, and the dissociatedoxygen removed from .contact with magnesium vapor.

The magnesium oxide and the carbon remain solid through the temperaturerange and to maintain contact both must be present in a finely dividedstate. When either magesium or calcium carbonate is very finely divided,a larger percentage of individually perfect crystals is found than whenthese materials are present in the massive form; and it has beenestablished by Faraday that perfect crystals refuse to efiioresce untilthe crystal face is fractured. Therefore, the dissociation does notproceed until the crystal is fractured by internal stress and thatrequires temperatures in the range of 2000 Fahrenheit, rather thanapproximately 1,600" Fahrenheit obtaining for the massive form.Therefore, the use of theflnely divided carbonate insures that CO: willbe released at approximately the temperature of MgO dissociation. Thecarbon dioxide when released will in turn substantially instantaneouslyreact with the carbon forming a mixture of diluent gases comprisingsubstantially carbon monoxide and a small amount of carbon dioxide.Hence the gases form currently with the release of. magnesiumvapors andremain in contact throughout.

Similar consideration is applied to the rate of magnesium oxidedissociation and it has been proved by Langmuir that the building up orbreaking down of a solid phase occurs only at the interface between thetwo phases. (The works of Faraday and Langmuir may be found in a"Treatise of Physical Chemistry, by H. S. Taylor, second edition, volume2, pages 1056 to 1069, inclusive.) Therefore, if merely the pure oxideand carbon be present, the dissociation occurs slowly, but if otherforeign solid particles are added to provide interfaces, the rate ofdissociation increases greatly. In the present process, the magnesiumoxide engages in an interface reaction with the undecomposed magnesiumcarbonate or the calcium carbonate present. Thus, while magnesiumcarbonate alone is operable, if the xide is used as a charge, anothersolid carbonate must be provided to obtain the desired interfacereaction. The result of the interface reaction and the presence of thediluent is shown bycurve C which indicates the actual temperature andpressure conditions at which dissociation will occur. This curve isbelow the others, and indicates that the dissociation is enhanced andthe reversibility reduced under the stated conditions.

It will be noted from Figure 1 that on dissociation the chamber containsth gaseous magnesium and; oxygen and removal of the latter is necessary.This is obtained by reaction with the carbon to form carbon monoxide orcarbon dioxide which is much less reversible at the temperaturesobtaining than oxygen, while the remainder of the oxygen forms an oxygenfilm on the carbon which is later released as the monoxide. The greaterthe amount which ultimately forms the monoxide, the lower will be thereversibility of the dissociation.

Much of the success of the process is due to the presence of the finelydivided carbonates and the formation of the diluent carbon dioxidesimultaneously in point of time and place with the dissociation of themagnesium oxide. Thus, as the oxide decomposes, carbon dioxide isimmediately formed, which greatly reduces the effective partial pressureof the oxygen to a point where it is considerably less than one-half ofthe dissociation pressure of the magnesium oxide. This lessens thereversibility proportionately and permits operation of the process atcorrespondingly lower temperatures and higher pressures as indicated bycurve C. Control of reversiblity through the partial pressure has longbeen sought, but, to the applicants knowledge, by the extraneousaddition of inert gases of one form or another to the system. Theformation of the desired diluent at the point of dissociation and as anessent al element of the dissociation is novel, not only reducing thepartial pressures, but as an integral part of the entire dissociationprocess.

In operation, the retort is evacuated continuously to remove the gasesformed. The portion containing the charge is heated to the hightemperatures required, while adjacent the outlet, the wall temperatureis maintained below the condensation point of magnesium vapor obtainingat that pressure. The vapor is then swept along by the evacuating gasesuntil it reaches the cooler wall on which it condenses in crystallineform while the CO: and C0 are disposed of through the vacuum pump. As apractical matter, a shallow charge is to be preferred as it is advisableto keep the path traversed by the gaseous magnesium through the chargeat a minimum to discourage reversion.

Summing up the process, the novel essentials are that solid interfacesbeprovided to hasten dissociation and that diluent gases be formed at thesame temperatures at which dissociation takes place. The former requiresthe presence of at least two solid components besides the magnesiumoxide to be dissociated. The latter requires the presence of a veryfinely divided carbonate which will not decompose until substantiallythe dissociation temperature of the oxide is reached.

Thus, both dissociation and reversibility are conversion is obtained atlow temperatures without.

the use of an expensive reducing agent such as ferrosilicon, heretoforethought necessary. The process is safe, since no explosive or dangerousgases other than CO need be handled and the magnesium is in thecrystalline form which is not pyrophorlc. The problems of operationpresent little difliculty since both temperature and pressure' arereadily controlled and the process may be operated in batch pots orcontinuously by charging the retort through vacuum locks in a.well-known manner.

We claim:

1. The process of producing reguline metallic magnesium comprising thesteps of mixing a magnesium-bearing compound, carbon and a foreignfinely divided crystalline component which remains solid at thetemperature attained,

briquetting such, mixture, subjecting such briquettes to temperaturesabove 2100 Fahrenheit at an absolute pressure not to exceed 3000microns, one of said materials decomposing to form carbon dioxide atsaid temperature and pressure, said carbon dioxide upon its releasebeing converted substantially to carbon monoxide exhaustin the gasesformed therein and thereby flowing the magnesium vapors released to acooler part of the system, and condensing said magnesium vapors underpressures and temperatures to obtain the metal in the solid, crystallineform.

2. The process of producing metallic magnesium from a mixture of finelydivided magnesium-bearing compound, carbon, and a crystalline materialdecomposing to form carbon dioxide upon heating under pressuressubstantially below atmospheric, which includes the step of dissociatingsaid magnesium-bearing compound and decomposing said source of carbondioxide said carbon dioxide being converted almost simultaneously tocarbon monoxide to reduce the partial pressure thereof.

3. The process of producing metallic magnesium which comprises the stepsof heating a finely divided and intimate mixture of magnesium oxide,calcium carbonate, and carbon to temperatures from 2100 to 2500Fahrenheit and at pressures not to exceed 1000 microns.

4. The process of producing metallic magnesium comprising the steps ofheating a finely divided and intimate mixture of partially calcineddolomite and carbon to a temperature of between 2100 and 2500 Fahrenheitat an absolute pressure not to exceed 1000 micons.

5. The process of producing metallic magnesium comprising the steps ofheating a finely divided and intimate mixture of magnesium carbonate,calcium carbonate and carbon at temperatures from 2100 to 2500Fahrenheit at an absolute pressure not to exceed 1000 microns.

6. In the process of producing metallic magnesium from the ores thereof,the step of providing an inert diluent gas evolved from the crystallinesubstance of the charge substantially simultaneously with the productionof the gaseous magnesium and intimately mixed therewith said reactionsoccurring at a temperature of 2100-2500" F. and at an absolute pressurenot to exceed 1000 microns.

'7. In the process of producing metallic magnesium from the oresthereof, the step of providing a diluent gas substantiallysimultaneously with the production of the gaseous magnesium, saiddiluent gas being formed by thermal decomposition of a crystallinecarbonaceous substance contained in the charge at substantially the sametemperature at which the vaporous magnesium is formed of a solidsubstance intimately mixed.

with said ore said reactions occurring at a temperature of 2100-2500 F.and at an absolute pressure not to exceed 1000 microns. v

8. In the process of producing metallicma'gnesium from the reaction ofthe carbon and the ores thereof, the step of providing diluent gasesresulting from the reaction of carbon dioxide gas and'said carbonsubstantially simultaneously with the production of the gaseousmagnesium, said diluent being formed by thermal decomposition of acarbonate at substantially the same temperature at which the vaporousmagnesium is formed, said carbonate being intimately mixed with said oresaid reactions occurring at a .temperature of 2100-2500 F. and at anabsolute pressure not to exceed 1000 microns.

9. In the process of producing metallic magnesium from the reaction ofcarbon and the ores thereof, the step of providing diluent gasesresulting from the reaction of carbon dioxide and a portion of saidcarbon substantially simultaneously with the production of the vaporousmagnesium, said diluent being formed by the thermal decomposition of acarbonate at substantially the same temperature at which the vaporousmagnesium is formed, said carbonate being intimately mixed with saidore, the normal decomposition temperature of said carbonate being raisedto substantially the magnesium formation temperature by reducing saidcarbonate to very small particles to increase the 'proportion of perfectcrystals therein said reactions occurring at a temperature of 2100-2500F. and at an absolute pressure not to exceed 1000 microns. 4

10. The process of producing metallic magnesium from the ores thereofwhich comprises the steps of partially calcining dolomite to obtaln amixture consisting substantially of magnesium oxide and calciumcarbonate, reducing the partially calcined material to a fine powder toincrease the proportion of perfect carbonate crystals therein andthereby increase the temperature of decomposition of said carbonate,mixing finely divided carbon with said calcined material, formingbriquettes of said mixture, placing said briquettesin a retort andapplying v heat and vacuum thereto, said temperature being suillceint todissociate said oxide and substantially simultaneously decomposing saidcarbonate, diluting the dissociated vaporous magnesium with the mixtureof carbon monoxide and carbon dioxide formed from the carbonate andflowing the vaporous magnesium thereby fromproximity of the briquettes,passing said vaporous magnesium while still under said reduced pressureover a cool surface to condense the magnesium in crystalline form, andremoving said diluent from said system.

11. The process of producing metallic magnesium from the ores thereofwhich comprises the steps of finely dividing a carbonate, the oxide ofwhich is characterized by remaining solid at the temperatureshereinafter attained in the system, said carbonate being dividedsufficiently to increase the proportion of perfect crystals therein andthe consequent temperature of decomposition thereof to at least 2000"F.,

intimately mixing magnesium xide and carbon with said carbonate, formingsaid mixture into briquettes, placing said briquettes in a retortsublected to temperatures in excess of 20001". and absolute pressurebelow 3000 microns, decomposingisaid carbonate to form a solid oxide and7 carbon dioxide, said carbon dioxide reacting with said carboniorming amixture of gases of substantially carbon monoxide and small amounts ofcarbon dioxide, dissociating said magnesium oxide to form vaporousmagnesium, said magnesium and said mixture of gases being formedsubstantially simultaneously, flowing said vaporous magnesium fromproximity of said briquettes by said mixture of gases,'passing saidvaporous magnesium whilestill subjectto said reduced pressure over a0001 surface to condense said magnesium to the solid crystalline state,and removing said diluent from said system.

12. A process for producing metallic magnesium which comprises the stepsof heating magnesium bearing compound in the presence of finely dividedcarbon and finely divided foreignciation by a second interfacialreaction releasing 5 a diluent gas which produces a vapor pressure ofthe magnesium at whichitcondenses without reasscciation, and condensingthe gaseous mag-' nesium so formed, both said interracial reactionsoccurring within the same range of, temperature.

HENRY FORD. ELBERT EDWIN ENSIGN. ARCHIBALD c. QUINN.

